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Isotopes What is an isotope? • Not every atom is exactly the same • We know that different elements have different number of protons. • However even atoms of the same element are not exactly the same. • Atoms of the same element can have different number of neutrons. These are called Isotopes. It’s all about the Mass Number • Because Isotopes differ in their number of neutrons, the mass of the atom also changes. • Isotopes are identified by their mass numbers. • Remember, mass number is the sum of what two particles? • Protons and Neutrons! • 1 Proton • 0 Neutrons • Mass # = 1 1 Proton 1 Neutron Mass # = 2 1 Proton 2 Neutrons Mass # = 3 Isotopic Notation • Isotopic Notation is generally how isotopes are written. • Mass number on top • Atomic number on bottom • This format makes it easy to calculate number of Neutrons • How do we calculate neutrons? • Isotopes have different mass numbers and also different numbers of neutrons. • Mass Number – Atomic Number = Neutrons You Try! - Part 3 • Input your answers in this order, no spaces. It should end up as one really big number!!!!! Protons, then Neutrons, and then Electrons • A) Carbon-12 • B) Carbon-14 • C) 104Be • D) Phosphorus-32 D. Average Atomic Mass • Weighted average of all isotopes based on their % abundance. • To determine, multiply the mass of the isotope by the decimal form of its abundance. Average Atomic Mass = (Mass * decimal) + (Mass * decimal) D. Average Atomic Mass • EX: Calculate the avg. atomic mass of oxygen if its abundance in nature is 99.76% 16O, 0.04% 17O, and 0.20% 18O. Average Atomic Mass = (16*.9976) + (17*.0004) + (18*.0020) = D. Average Atomic Mass • EX: Find chlorine’s average atomic mass if approximately 8 of every 10 atoms are Chlorine-35 and 2 are Chlorine-37. Avg. Atomic Mass (35*.8) + (37*.2) = 35.4 amu Ions • • • • • Atoms are identified by which particle? PROTONS! Different Isotopes have different? NEUTRONS! So, which particle do you think identifies Ions??? • ELECTRONS! Ions • Ions form when an atom gains or loses electrons. • Why do they do that? • Atoms gain or lose electrons in order to reach a stable configuration. We will talk more about that in a few slides. Ions • Let’s look at a normal atom of Lithium. • Protons? • OOO • If the atom is neutral, how many electrons? • o o o • Can we ever change the number of protons and still have a Lithium Atom? •O O O • What if the atom lost an electron to become stable. •o o • Is the atom still neutral? • What would be the charge of this atom now? • Can we ever change the number of protons and still have a Lithium Atom? •O O O • What if the atom gained an extra electron to become stable. •o o o o • Is the atom still neutral? • What would be the charge of this atom now? In review…. • If an atom loses electrons, the overall charge of the atom becomes positive. Positive because there will be more protons than electrons. • If an atom gains extra electrons, the overall charge of the atom becomes negative. Negative because there will be more electrons than protons. So what if you see this? • An atom of Nitrogen has a -3 charge. • What happened? • How many electrons does this ion have? • An atom of Magnesium has a +2 charge. What happened? • How many electrons does this ion have?